The present invention relates to improvements in particle classifiers, which separate coarse particles from a stream of gas entrained with a mixture of coarse and fine particles, having several outlet conduits for discharging gas and fine particles. This invention relates particularly to a fuel classifier and a method for separating coarse fuel particles from a mixture of fine and coarse fuel particles entrained in an air stream and returning larger fuel particles to a pulverizer for further size reduction. Meanwhile, the air stream carrying the fine fuel particles can be used for firing a boiler.
Industrial and utility-sized coal boilers may be equipped with two to several dozens of coal burners to deliver fuel to a combustion furnace. The number of burners depends on the size of the boiler and the configuration of the furnace. Commonly-used burner configurations include single wall firing, opposed wall firing, and tangential firing. Coal is typically pulverized in a mill, e.g., a spindle mill or a ball mill, to a fineness that is suitable for combustion in the furnace. Large boilers include several pulverizers, each of which delivers fuel and primary air to a set of burners. One requirement for efficient combustion and low emission levels is that equal or controlled quantities of fuel be delivered to each of the separate burners.
A pulverizer is usually combined with an aerodynamical classifier, which imparts a swirling motion to a coal-air mixture discharged from the mill and centrifugally separates the coal fines from the coarse product. Classifiers operated in a positive air pressure usually have multiple outlet conduits through which coal fines are transported by a flow of primary air to multiple burners in the boiler. The coarse material is returned to the mill and re-ground.
In order to observe stringent environmental regulations and to achieve efficient boiler operation, the flows of coal and air from the classifier to each burner must be precisely controlled. Generally, the air distribution can be balanced quite easily by adjusting the flow impedances of the various coal-air lines. The coal flow, on the other hand, is more difficult to control since it is dependent, in a complicated way, on the conditions within the pulverizer, classifier, and fuel lines, including the burners.
The distribution of the coal flow between the various outlet conduits in a classifier can be improved by increasing the homogeneity of the pulverized coal in the vicinity of the classifier outlets. This can be achieved by improving the pulverizer performance or by optimizing the geometry of the classifying blades. Japanese Patent Publication JP 63259316 A2, for example, discloses a coal distributor wherein a swirling solid-gas flow is transformed by radial vanes into a vertically uprising flow which collides with a horizontal plate so as to achieve a uniform particle concentration. Despite this and other like measures to provide a homogeneous coal distribution, the coal tends to turn stratified in the classifier, resulting in flow variations as large as 20% among the various outlets.
U.S. Pat. No. 4,540,129 discloses a pulverizer wherein each of the multiple lines between the pulverizer and a set of coal burners includes a valve to control the flow rates of coal and primary air. This commonly-used method controls the coal and air streams simultaneously, but does not make it possible to affect the coal flow irrespective of the air flow. Due to the different characteristics of air and coal streams, there are often situations where a coal stream needs to be adjusted independently of the air stream.
It is also known to arrange adjustable guide vanes at the outlets of each coal line in a classifier. The guide vanes either capture the pulverized coal or divert it from the outlet. These vanes, however, also impede the flow of primary air, and thus affect both the air and the coal flow. Therefore, such vanes have only limited potential for balancing an asymmetrical coal distribution within a classifier.
An object of the present invention is to provide an apparatus and method for achieving efficient and environmentally advantageous operation of a pulverized fuel fired boiler.
More particularly, an object of the present invention is to provide a particle classifier and a method for utilizing the particle classifier in order to balance the distribution of pulverized fuel among multiple outlet conduits of the particle classifier.
A further object of the present invention is to provide a new particle classifier and a method for utilizing the particle classifier in order to balance the distribution of pulverized fuel among the multiple outlet conduits of the particle classifier while minimizing the effect on the primary air flow distribution.
Another object of the present invention is to provide a new particle classifier and a method for utilizing the particle classifier in order to maintain a balanced distribution of pulverized coal among the multiple outlet conduits of the particle classifier in various process conditions.
In one aspect, the present invention relates to a classifier for separating coarse particles from a stream of gas and a mixture of coarse and fine particles. The classifier includes a generally cylindrical outer casing including a vertically-oriented side wall and an upper head, a generally conical inner casing provided within the outer casing and configured so as to provide an annular passageway between the inner casing and the side wall of the outer casing through which the stream of gas and particles can flow upwardly, a plurality of angled circumferentially-spaced vanes supported by the upper head of the outer casing for imparting rotational motion to the stream of gas and particles so as to separate coarse particles from the mixture of coarse and fine particles, and an outlet chamber at an upper portion of the inner casing. The outlet chamber includes (i) a top plate with a plurality of outlet openings for discharging streams of gas and fine particles from the classifier, and (ii) at least one pivotable distribution vane for controlling the distribution of the fine particles between each of the outlet openings by affecting the rotational movement of the stream of gas and particles.
In another aspect, the present invention relates to a method for separating coarse particles from a stream of gas and a mixture of coarse and fine particles in a classifier. The method includes (a) passing the stream of gas and particles upward through an annular passageway between a side wall of a generally cylindrical outer casing and a generally conical inner casing, (b) imparting rotational motion to the stream of gas and particles so as to separate coarse particles from the mixture of coarse and fine particles by passing the stream of gas and particles through a plurality of angled circumferentially-spaced vanes attached between an upper edge of the inner casing and an upper head of the outer casing, and (c) discharging streams of gas and fine particles through a plurality of outlet openings in a top plate of an outlet chamber at an upper portion of the inner casing. In step (c) the rotational movement of the stream of gas and particles in the outlet chamber is affected by adjusting the pivot angle of at least one pivotable distribution vane arranged in the outlet chamber so as to control the distribution of fine particles between each of the outlet openings.
In still another aspect, the present invention relates to an apparatus for separating coarse particles from a stream of gas entrained with a mixture of coarse and fine particles. The apparatus includes an outer casing, an inner casing disposed within the outer casing and configured to define a passageway between the outer casing and the inner casing through which the stream of gas and mixture of coarse and fine particles can flow substantially upwardly, a plurality of angled vanes for imparting a rotational flow to the stream of gas and particles as the stream passes from the passageway to within the inner casing in order to separate the coarse particles from the fine particles entrained within the stream of gas, a plurality of outlets for discharging the stream of gas and fine particles from the apparatus, and at least one distribution vane pivotably mounted with respect to the outlets for controlling the distribution of fine particles among the various outlets by affecting the rotational flow of the stream of gas and fine particles.
In a further aspect, the present invention relates to a method of using a classifier to separate coarse particles from a stream of gas entrained with a mixture of coarse and fine particles. The method includes (a) imparting rotational movement to the stream of gas and particles by passing the stream through a plurality of angled vanes, (b) separating, by centrifugal and gravitational force, the coarse particles from the fine particles entrained within the stream of gas, (c) discharging the stream of gas entrained with the fine particles from a plurality of outlets in the classifier, and (d) controlling, by adjusting at least one distribution vane in a way that affects the rotational movement of the stream of gas and fine particles within the classifier, the distribution of the fine particles to the outlets.
Coal mill classifiers typically include one or more sets of vanes, which induce a swirling or rotational motion in the coal-air stream and bring about centrifugal separation of coarse coal particles from the stream. The downstream portion of the classifier comprises an outlet chamber, or space, for distributing the fine coal between the various outlet conduits of the classifier. Usually, the outlet space is located symmetrically at the top portion of the classifier and is provided with a coal inlet conduit at its vertical symmetry axis. The outlet space is restricted by a cylindrical or conical side wall and an annular top plate including a plurality of coal outlets. The side wall may be a solid wall or it may comprise static or rotatable vanes. The outlet space generally does not include means for further enhancing the swirling of the coal-air stream.
The distribution vanes are arranged in the outlet space to cause balanced coal distribution to the various outlets. They can be used for partially disrupting the swirling pattern of the air-coal mixture in the outlet space and thereby producing additional mixing and more homogenous coal distribution. More particularly, the distribution vanes can be used for balancing the coal flow between the various outlets by, as required, decreasing or increasing the coal flow to one or more of the outlets. In order not to adversely affect the air flow, the distribution vanes are generally arranged in positions that are spaced apart from the outlet openings.
Typically, the annular top plate of the outlet space comprises circular inner and outer zones and a circular zone between the inner and outer zones including symmetrically distributed coal outlets, hereinafter referred to as the circular intermediate zone. It is also possible that the outlets abut the central coal inlet conduit and/or the outer edge of the top plate, in which case there would be no inner and/or outer zone. The areas in the intermediate zone between the coal outlets are hereinafter referred to as the xe2x80x9cintermediate free areas.xe2x80x9d
The distribution vanes are, according to a preferred embodiment of the present invention, arranged below the intermediate free areas. There may be only one distribution vane located below a selected free area, or there may be multiple vanes arranged below the various free areas. According to a preferred embodiment of the present invention, there is a distribution vane located below the free areas between each of the coal outlets.
According to another preferred embodiment of the present invention, the distribution vanes are arranged below the outer zone. In this embodiment, preferably one or more distribution vanes are arranged radially outside the intermediate free areas, but they can also be arranged radially outside the areas of the coal outlets.
According to a third preferred embodiment of the present invention, the distribution vanes are arranged below the inner zone. In this embodiment, preferably one or more distribution vanes are arranged radially inside the intermediate free areas, but they can also be arranged radially inside the areas of the coal outlets.
In order to balance coal flow distribution among the various outlets, the orientations of the distribution vanes are adjustable, preferably individually adjustable. In some cases an initial adjustment may be sufficient for eliminating flow maldistributions, but preferably the distribution vanes are equipped with means, such as a crank operated by a hydraulic or pneumatic piston, for adjusting the vane orientations externally, whenever needed. There may be a controller and/or user interface linked between the means for adjusting the vane orientations and devices measuring coal flow in different outlet conduits of the classifier.
According to a preferred embodiment of the present invention, each distribution vane is pivotably connected to a vertical shaft attached to the top plate of the outlet chamber. In some geometries it may also be possible to attach the shafts to the side wall or to a bottom plate of the outlet chamber. The shafts are preferably connected to the leading edges or to the central parts of the vanes.
The distribution vanes are preferably arranged in the upper part of the outlet space. The lengths and heights of the vanes are typically between about 50% and about 150% of the diameter of the outlet conduits. According to a preferred embodiment of the present invention, the vanes can be pivoted from the original orientation along the swirling flow direction to a maximal pivot angle transverse to the flow direction. Preferably, in the maximal tilt orientation the vanes cover between about 30% and about 70% of the corresponding vertical cross section of the upper part of the outlet space. According to another preferred embodiment of the present invention, the vanes can be pivoted in both directions, e.g., from about +45 degrees to about xe2x88x9245 degrees from their original orientation.
When all the distribution vanes are oriented along the local swirl, they have a relatively insignificant effect on the swirling flow. When one or more of the vanes partially or completely traverse the swirl, however, the vanes redirect the coal flow and enable control of the coal distribution. The air flow is much less affected by the distribution vanes than the coal flow. Therefore, it is possible to balance the coal flow through adjustment of the distribution vanes while minimizing the effect on the primary air flow. Nor do the distribution vanes of the present invention cause any significant pressure loss.
The present invention thus improves the combustion process in the furnace by decreasing the amount of unburned carbon in the ash. Also, the emission levels, especially the NOx-emissions, are reduced by the improved stoichiometric ratio of fuel and air provided to the burners. Controlling the air and fuel balance at the burners also results in improved boiler oxygen and steam temperature profiles.
The present invention can be applied to all types of vertical spindle mills and other mill types that utilize the aerodynamical classifiers commonly found on vertical spindle mills. It can be used with static as well as rotating classifiers equipped with several coal outlets and, thus, having a need to balance the coal flow between the various outlets.
A better understanding of these and other objects, features, and advantages of the present invention may be had by reference to the drawings and to the accompanying description, in which preferred embodiments of the invention are illustrated and described.